1. Field of the Invention
The present invention relates to a method of controlling the Bragg wavelength and a method of compensating for temperature dependency of the Bragg wavelength of a waveguide grating, and a waveguide grating device using the same.
2. Description of the Related Art
In a wavelength division multiplexing(WDM) system, an optical multiplexer/demultiplexer that combines or splits wavelength-multiplexed signals, a band pass filter and a dispersion compensator that compensates for dispersion of an optical fiber that transmits signals are key devices, with improvements in the characteristics thereof being viewed as important issues.
Recently, particularly as advancements in the WDM systems have been made, such needs have been mounting to improve the Bragg wavelength control technology and improve the temperature characteristic of the Bragg wavelength of an optical multiplexer/demultiplexer that combines or splits wavelength-multiplexed signals, a band pass filter and a dispersion compensator.
FIG. 25 shows an example of a band pass filter of the prior art that utilizes a waveguide grating. This waveguide grating has a function of reflecting light of a particular wavelength, with the Bragg wavelength xcexB being defined by the following formula (1), where neff denotes the effective refractive index of the waveguide grating, and xcex9 denotes the period of the waveguide grating.
xcexB/neff2xcex9xe2x80x83xe2x80x83(1)
In the band pass filter shown in FIG. 25, light incident on a port P1 is divided into two portions by a 3 dB coupler 105. Light of the same wavelength as the Bragg wavelength xcexB of a grating 102, namely the signal light, is reflected from the grating 102 and synthesized in the 3 dB coupler 105 again and is output from the port P2. Light having a wavelength different from the Bragg wavelength xcexB of the grating, namely noise light, is transmitted through the grating 102 and is output from ports P3, P4. Thus the band pass filter of FIG. 25 is capable of cutting off the noise light and extracting only the signal light from the port P2.
FIG. 26 shows an example of a multiplexer/demultiplexer that utilizes the waveguide grating. In the multiplexer/demultiplexer of the prior art shown in FIG. 26, for example, when signals of different wavelengths (xcex1, xcex2, xcex3, . . . ) enter a port P1, only the signal light of a particular wavelength (xcex1 in the drawing) is reflected from the grating 102 and is output from the port P2 (demultiplexing). Signal light of other wavelengths (xcex2, xcex3, . . . ) is output from the port P4. Signal light having a particular wavelength (light having wavelength xcex1) that has entered the port P3 is reflected from the grating 102 and is output from the port P4 (multiplexing).
FIG. 27 shows an example of a dispersion compensator that utilizes the waveguide grating. In optical communication, since an optical fiber used as a medium for transmitting optical signals experiences dispersion due to the refractive index changing depending on wavelength, propagation speed along the propagation path varies depending on the wavelength of the light, thus resulting in different amounts of group delay for different wavelengths. Since an optical pulse signal has a certain spread in wavelength, the pulse signal is broadened as shown in FIG. 28B after having been propagated over several tens of kilometers, due to the difference in group delay by the wavelength (FIG. 28A shows signal waveform before propagation and FIG. 28B shows signal waveform after propagation). Therefore, ultra-high speed optical communication requires a technique for compensating for the dispersion in the optical fiber.
Relation between wavelength and group delay during propagation through an optical fiber is shown in FIG. 29A, with the dispersion that is given by differentiation thereof being shown in FIG. 29B. Compensating for this effect requires a device that has such group delay and dispersion characteristics as shown in FIGS. 30A, 30B, which can be achieved by means of a dispersion compensator shown in FIG. 27. The waveguide grating 102 of the dispersion compensator is formed so that the period thereof becomes longer as the distance from the input port increases. With such a configuration that the period changes as described above, reflecting position changes with the wavelength of the light thus resulting in such a gradient in group delay as shown in FIG. 30A. That is, dispersion remains a constant and positive value as shown in FIG. 30B, thus making it possible to compensate for the dispersion because of the sign of this dispersion reverse to the dispersion of the optical fiber.
FIG. 31 shows an example of a variable dispersion compensator that utilizes a waveguide grating. Since the amount of dispersion that a dispersion compensator is required to compensate varies, depending on the length of an optical fiber used as the propagation path and on the conditions of operation, when a dispersion compensator of a fixed amount of dispersion to be compensated is used, the optimum dispersion compensator has been selected by preparing a plurality of dispersion compensators of different amounts of dispersion and inserting them successively. The variable dispersion compensator shown in FIG. 31, by contrast, is capable of making a fine adjustment of the dispersion and therefore makes it possible to solve the problem of the trials and errors being required to determine the optimum dispersion compensator. Specifically, in the variable dispersion compensator shown in FIG. 31, heating means 24a, 24b apply heat to the optical waveguide. At this time, a temperature gradient is generated in the waveguide grating by differentiating the calorific values generated by the heating means 24a and the heating means 24b. 
The effective refractive index neff of the waveguide grating is a function of temperature. Consequently, the temperature gradient generated by applying different amounts of heat from the heating means 24a and the heating means 24b causes a gradient in the effective refractive index neff of the optical waveguide along the longitudinal direction of waveguide. With this configuration, since the Bragg wavelength becomes a function of neff as shown by formula (1), the Bragg wavelength changes with the position along the longitudinal direction of the waveguide grating. As a result, the temperature gradient gives an effect similar to the so-called chirped grating, so that the distance from the reflecting point differs depending on the wavelength, thereby causing dispersion. With this configuration, relations of the temperature gradient with the group delay and the dispersion become as shown in FIGS. 32A, 32B, and 32C, thus making it possible to adjust the dispersion according to changes in the temperature gradient generated by the heating means.
Now a method of producing the waveguide grating used in the optical devices described above will be described. Typical methods of producing the waveguide grating include one that utilizes light-induced change in refractive index. The light-induced change in refractive index refers to the change that occurs in the refractive index of a silica-based optical waveguide, that is doped with germanium, when irradiated with ultraviolet rays. Specifically, when two rays of ultraviolet light are incident on the waveguide and interfere with each other and form a fringe pattern, the refractive index of the waveguide changes according to the period of the fringe pattern, thus generating a waveguide grating. Though the change in the refractive index is as small as on the order of 0.001, the grating has a very small period of about 500 nm and therefore a waveguide grating of around 20000 periods can be produced with a length of about 1 cm, thus easily achieving about 100% reflectivity at the Bragg wavelength.
The waveguide gratings produced as described above have variations in the grating period due to variability in the production process, resulting in some variations in the Bragg wavelength of the grating. The Bragg wavelength also becomes greater when the intensity and/or irradiation time of the ultraviolet radiation increases or the waveguide grating is formed in a direction deviated from the direction perpendicular to the propagating direction of light through the waveguide due to the producing facility or other condition during production of the waveguide grating. Thus a process of adjusting the Bragg wavelength is required. In the prior art, such methods have been employed to adjust the Bragg wavelength as measuring the Bragg wavelength after forming the waveguide grating and, in case the measured Bragg wavelength is different from the desired value, the waveguide grating is further irradiated with ultraviolet light (Unexamined Patent Publication (Kokai) No. 9-288205) or applying heat (Unexamined Patent Publication (Kokai) No. 10-339821).
The effective refractive index neff of the optical waveguide made of silica-based material also changes with temperature, and the Bragg wavelength xcexB accordingly changes following the formula (1) as shown in FIG. 32C. Since a grating formed on a silica-based waveguide normally has a refractive index that changes at a rate of 0.01 nm/xc2x0 C., some measures must be taken to prevent the Bragg wavelength from changing with the temperature change in the band pass filter, the multiplexer/demultiplexer and the dispersion compensator. Since the problem of the Bragg wavelength changing with temperature occurs due to the temperature characteristic of the material, the Bragg wavelength has been prevented from changing in the prior art by such methods as controlling the temperature by means of a Peltier element or the like, or covering the waveguide with an optical medium that has an inverse temperature dependency to that of the refractive index of the waveguide (Unexamined Patent Publication (Kokai) No. 10-186167). At the general conference of Electronic Communications Engineering Association, 1999, Arai et al. gave a report titled xe2x80x9cTemperature characteristic compensation for waveguide type Add/Drop filterxe2x80x9d (C-3-98), proposing such techniques as bonding on a substrate a metal plate or the like that has a higher thermal expansion coefficient than the substrate including the waveguide.
However, such methods as re-irradiating the waveguide grating with ultraviolet light and applying heat have problems such as taking a long time for adjustment and being incapable of being produced at a low cost.
As to the configuration for improving the temperature characteristic (decreasing the change in Bragg wavelength with temperature change), there has been a problem of complicated configuration and long period of time taken in production, in any of the methods of controlling the temperature by means of a Peltier element or the like, covering the waveguide with an optical medium that has an inverse temperature dependency to that of the refractive index of the waveguide and bonding on the substrate a metal plate or the like having higher thermal expansion coefficient than the substrate including the waveguide.
Accordingly, a first object of the present invention is to provide a waveguide grating device that allows it to control the Bragg wavelength more easily than in the prior art after production.
A second object of the present invention is to provide a waveguide grating device capable of reducing the change in the Bragg wavelength due to temperature change.
A third object of the present invention is to provide a method of adjusting the Bragg wavelength of a waveguide grating device that allows it to control the Bragg wavelength easily after production.
In order to achieve the object described above, a first waveguide grating device of the present invention comprises an optical waveguide formed on a substrate and a waveguide grating formed in a portion of the waveguide, wherein bending stress applying means is provided for bending at least a portion of the substrate whereon the waveguide grating is formed.
With this configuration, since the portion of the substrate whereon the waveguide grating is formed can be bent by means of the bending stress applying means, it is made possible to change the grating period of the waveguide grating thereby changing the Bragg wavelength thereof.
Thus the grating period of the waveguide grating can be changed by bending the portion where the waveguide grating is formed after forming the waveguide grating.
Therefor the waveguide grating device of the present invention is capable of changing the Bragg wavelength after forming the waveguide grating, the Bragg wavelength can be controlled more easily than in the prior art.
In the first waveguide grating device of the present invention, in case the substrate is housed in a casing, the substrate can be fastened in the casing on both sides of the waveguide grating by means of substrate support bodies, and the bending stress applying means may be substrate pressing means that is installed in the casing so as to make contact with the substrate above the waveguide grating and is movable in the direction perpendicular to the substrate.
With this configuration, the Bragg wavelength can be controlled easily with a simple constitution.
In the first waveguide grating device described above, distance L1 between the substrate support bodies is preferably set so as to satisfy a relation of inequality t1/L1 less than 0.2 with t1 representing the thickness of the substrate, which makes it possible to effectively change the Bragg wavelength of the waveguide grating.
With this configuration, it is made possible to effectively change the Bragg wavelength of the waveguide grating and easily control the Bragg wavelength.
Further in the first waveguide grating device of the present invention, it is preferable that the substrate pressing means has an arc shape with the center thereof located at a point, or an arc-shaped end portion having an axis lying on a straight line that is parallel to the substrate, which makes it possible to prevent the substrate from breaking. Therefore it is possible to improve the reliability.
In the first waveguide grating device of the present invention, the substrate pressing means may also be provided so as to make contact with the surface of the substrate whereon the waveguide grating is formed.
In this configuration, the Bragg wavelength can be decreased by pressing.
Also in the first waveguide grating device of the present invention, the substrate pressing means may also be provided so as to make contact with the surface of the substrate opposite to the surface thereof whereon the waveguide grating is formed.
In this configuration, the Bragg wavelength can be increased by pressing.
Also in the first waveguide grating device of the present invention, the bending stress applying means may be a stress applying plate that is connected to the surface of the substrate so as to oppose the waveguide grating and has a thermal expansion coefficient different from that of the substrate, with the stress applying plate being connected to the surface of the substrate whereon the waveguide grating is formed.
With this configuration, the constitution can be made compact and simple since the bending stress applying means can be made without using the casing.
Also in the first waveguide grating device of the present invention, the substrate pressing means may be an actuator that expands and contracts in response to a voltage applied thereto.
With this configuration, the Bragg wavelength can be controlled electrically.
Further in the first waveguide grating device of the present invention, a first portion of the substrate where the waveguide grating is formed is preferably made thinner than the other portion. This configuration makes it possible to generate a relatively large bending stress in the substrate with a relatively weak force.
Therefore it is possible to control the Bragg wavelength efficiently.
Also in the first waveguide grating device of the present invention, the waveguide grating can be formed so that the grating period thereof changes progressively.
With this configuration, the waveguide grating device can function as a dispersion compensator.
Therefore it is made possible to provide the dispersion compensator capable of controlling the Bragg wavelength more easily than the prior art after production.
The method of adjusting the Bragg wavelength of the waveguide grating according to the present invention is a method of adjusting the Bragg wavelength of the waveguide grating formed in a portion of an optical waveguide that is formed on the substrate, wherein a bending stress is applied so as to bend at least a portion of the substrate where the waveguide grating is formed, thereby changing the grating period of the waveguide grating.
With this configuration, since the portion of the substrate whereon the waveguide grating is formed can be bent by the bending stress applying means, grating period of the waveguide grating can be changed so that the Bragg wavelength can be changed.
Therefore it is possible to change the Bragg wavelength easily.
A second waveguide grating device of the present invention comprises a substrate, whereon a waveguide grating and an optical waveguide connected to the waveguide grating are formed, that is supported in a casing on one of principal planes of the substrate by using substrate support bodies, wherein substrate pressing means is installed in the casing so as to make contact with the other principal plane of the substrate in such a condition as being movable in a direction perpendicular to the substrate or being fixed, while the thermal expansion coefficient of either the substrate support body or the substrate pressing means, or alternatively the thermal expansion coefficients of both the substrate support body and the substrate pressing means are set so that the thermal expansion coefficient cancels the change in the Bragg wavelength of the waveguide grating caused by a change in temperature.
With this configuration, temperature compensation can be made with a simple constitution.
In the second waveguide grating device of the present invention, it is more preferable that the substrate support body has an end face of arc shape having radius of curvature with an axis lying on a straight line that is parallel to the substrate so as to alleviate the concentration of load, while making linear contact with the substrate on the end face.
With this configuration, the substrate can be prevented from breaking and reliability can be improved further.
Also in the second waveguide grating device of the present invention, it is preferable that end portion of the substrate pressing means has arc shape having radius of curvature with center thereof located on one point or an axis lying on a straight line that is parallel to the substrate so as to alleviate the concentration of load.
In this configuration, since concentration of the load on the substrate can be alleviated by forming the end portion of the substrate pressing means in arc shape having radius of curvature having center on one point or an axis lying on a straight line that is parallel to the substrate, the substrate can be prevented from breaking and reliability can be improved.
In the second waveguide grating device of the present invention, substrate pressing means may also be provided in plurality. In this case, the substrate pressing means are preferably installed on both sides of the waveguide grating, which makes it possible to substantially uniformly distribute the stress generated in the portion of the substrate where the waveguide grating is formed under a load.
Therefore deterioration of the characteristic due to uneven stress distribution can be prevented.
Also in the second waveguide grating device of the present invention, the substrate pressing means may be provided, on the end portion thereof, a load splitting member having first and second end faces that make linear contact with the substrate and are parallel to each other, with the first and second end faces straddling the waveguide grating. With this configuration, it is also made possible to substantially uniformly distribute the stress generated in the portion of the substrate where the waveguide grating is formed under a load. With this configuration, it is possible to prevent deterioration of characteristic from occurring due to uneven stress distribution.
Also in the second waveguide grating device of the present invention, the substrate pressing means may also be provided, at an end thereof, with a flat plate having a top surface and a bottom surface that are parallel to each other. With this configuration, too, it is made possible to substantially uniformly distribute the stress generated in the portion of the substrate where the waveguide grating is formed under a load. Therefore it is possible to prevent deterioration of characteristic from occurring due to uneven stress distribution.
Further in the second waveguide grating device of the present invention, the surface of the substrate whereon the waveguide grating is formed is preferably provided with a organic material layer. In this configuration, the reliability can be improved further.
Also in the second waveguide grating device of the present invention, the substrate may also be hermetically sealed in the casing instead of providing the organic material layer. In this configuration, the reliability can be improved also by hermetically sealing the substrate in the casing instead of providing the organic material layer.